I was starting to wonder whether anyone here knew Cortex+ at all.
I haven't run Leverage, but have a bit of experience with Marvel Heroic RP. My prep for those sessions generally means thinking about some opponents to use, and thinking through some likely locations and Scene Distinctions for them. (And when I'm improvising, which is often, what I generally do is come up with some Scene Distinctions and flip through my books to find some opponents.)
In the last session I ran, the PCs were planning to break into the Latverian Embassy (looking for Mariko Yashida, who seemed to have been kidnapped). They decided to go to the urban planning department before their break in, looking for information eg about sewers and other possible secret entrances. Instead of running this as a Transition Scene (which probably would have made more sense) I ran this as an Action Scene (don't ask why - it sort of unfolded in a not-fully-thought-through way). The "action" ended up being the creation of assets (sewer plans, from memory; maybe something else I've forgotten) via rolls against the Doom Pool. I think the "sewer plan" asset then served as a bonus die when half the team took their first suite of actions during the break in (which, from memory, was taking out the basement guards in the Embassy).
Assets: whomever it's the asset to gets the die... EG: Nar is carrying a 30mm AMR (a Big Big gun)... and trying to hid in a sniper blind. Nar has taken the 30mm AMR as a signature item asset. The blind is a scene asset. The guy searching for Nar can treat it as an asset in finding him (they're HUGE), and once he's found it, use the blind as an asset in sneaking up on him.
At least in MHRP, only the character who was intended to benefit from the asset (either the character who created it, or the character who was being supported) gets to use it in their pool.
So in MHRP, in the situation you describe, if the GM wants the guy searching for Nar to get a benefit from the fact that Nar's gun is BIG, the GM has to spend a die from the Doom Pool to reflect this. Or maybe create a complication ("Your gun is really TOO big!") and then use that.
But I dunno how other Cortex+ systems handle this.
What happens when a player is rolling to convince an NPC of something, and the NPC has a trait that's relevant but should work in the PC's favor? Do I roll it as part of the stakes or not? For example, if the Mark has the trait "Greedy D8," and the PC appeals to his greed, how is that reflected in the roll?
In MHRP, "Greedy" would likely be a Distinction, and the GM would include it in the pool at d4 rather than d8.
Alternatively, many characters (NPCs as much as PCs) have Limits which allow a power to be treated as a complication. So a player could spend a Plot Point to trigger the Limit, turning the power into a complication, and hence adding it to his/her pool. (In MHRP there don't tend to be powers like Greedy - but maybe a telepath could have this sort of Limit, reflecting the fact that their "open" mind makes them more than typically vulnerable to other telepaths.)
I'm not sure how well the above approaches translate to Leverage.